This invention relates generally to electric energy conversion, and, more specifically, to a system and a method for low voltage ride through capability of small synchronous generators with low moments of inertia connected to a power grid.
In traditional electric power systems, most of the electrical power is generated in large centralized facilities, such as fossil fuel (coal, gas powered), nuclear, or hydropower plants. These traditional plants have excellent economies of scale but usually transmit electricity long distances and can affect to the environment. Distributed energy resource (DER) systems are small power generators (typically in the range of 3 kW to 10,000 kW) used to provide an alternative to or an enhancement of traditional electric power systems. Small power generators may be powered by small gas turbines or may include fuel cells and/or wind powered generators, for example. DER systems reduce the amount of energy lost in transmitting electricity because the electricity is generated very close to where it is used, perhaps even in the same building. DER systems also reduce the size and number of power lines that must be constructed. However, due to increased use of small generators, some utilities are now requiring that small generators provide enhanced capabilities such as fault voltage ride through.
When a fault in the utility system occurs, voltage in the system could decrease by a significant amount for a short duration (typically less than 500 milliseconds). Faults can be caused by at least one phase conductor being connected to ground (a ground fault) or by the short circuiting of two or multiple phase conductors. These types of faults occur during lightning and wind storms, or due to a transmission line being connected to the ground by accident. For the purposes of this specification, the term “fault” is intended to cover significant voltage reduction events. The term “fault” as used herein, is intended to cover any event on the utility system that creates a momentary reduction or increase in voltage on one or more phases. In the past, under these inadvertent fault and large power disturbance circumstances, it has been acceptable and desirable for small generators to trip off line whenever the voltage reduction occurs. Operating in this way has no real detrimental effect on the supply of electricity when small generator power penetration is low. However, as penetration of small generators on the grid increases, it is desirable for a small generator to remain on line and ride through such a low voltage condition and even more important to stay in synchronism, being able to generate energy after the fault is cleared. This new operation is similar to the requirements applied to traditional generating sources such as fossil fueled synchronous generator plants.
Therefore, it is desirable to determine a method and a system that will address the foregoing issues.